1. Field of the Invention
This invention relates to photovoltaic elements useful for converting light and particularly for converting solar energy into electrical energy. The invention features the use of organic compounds.
2. State of the Prior Art
So-called Schottky barrier or P-N junction photocells rely upon the fact that a built-in potential exists at the metal/semiconductor interface as in the Schottky device or at the junction between the P-type and N-type semiconductors as in the P-N junction device. Electron-hole pairs generated by the absorption of light in the semiconductor are separated due to the built-in field at the interface, establishing an electrical potential.
Among chief materials used in the past for solar cells have been inorganic semiconductors, due to their fairly high conversion efficiencies which have been as high as 12 to 15 percent, for example, for silicon. However, such devices have proven to be very expensive to construct, due to the melt and other processing techniques necessary to fabricate the semiconductor layer. As a result, such devices have had extensive practical utility only in the field of space exploration, and not in terrestrial applications.
In an effort to reduce the cost of solar cells, organic photoconductors and semiconductors have been considered, due to their inexpensive formation by solvent coating and similar techniques. However, prior art organic materials have generally produced solar cells with conversion efficiencies only as high as about 0.05 percent at their highest, when exposed to incident sunlight at an intensity of 100 mW/cm.sup.2. An example of such a material is crystal violet, as described, for example, in U.S. Pat. No. 3,844,843. Still higher efficiencies at least as high as 0.1 percent are desirable if the cells are to have practical terrestrial use, notwithstanding their inexpensive cost of manufacture. An efficiency of 0.3 percent was reported as being achieved through the use of an undisclosed dopant, as noted in "Prospects for Direct Conversion of Solar Energy to Electricity," AWA Technical Review, Volume 15, No. 4, 1974, footnote 3, but none of the materials used has been disclosed.
Solar cells utilizing other organic photoconductive materials are disclosed in U.S. Pat. Nos. 3,009,006; 3,057,947; 3,507,706; 3,530,007; and IBM Technical Disclosure Bulletin 18 (8), page 2442 (January 1976). However, there is no disclosure in any of these publications how to manufacture a solar cell which exhibits a conversion efficiency high enough for extensive practical terrestrial use, i.e., greater than about 0.1 percent.
Multilayer photoconductive compositions have been formulated in the past, for xerographic application, using porphyrinic compounds overlayered with a charge-transport layer, as disclosed, for example, in U.S. Pat. Nos. 3,895,944 and 3,992,205. However, such charge-transport layers in U.S. Pat. No. 3,895,944 have required the use of binders, as well as sensitizers, and in U.S. Pat. No. 3,992,205 the layer containing phthalocyanine requires the use of another pigment admixed therewith.
Phthalocyanine, a porphyrinic compound, has been used in organic solar cells in the past, in contact with a layer of electron acceptors such as oxidized tetramethyl p-phenylenediamine, .beta.-carotene, dibrominated p-phenylenediamine, p-chloranil and the like. Examples are illustrated in U.S. Pat. No. 3,057,947. However, such cells have extremely low conversion efficiencies, less than 10.sup.-7 percent (power output, col. 3, line 69, divided by 100 milliwatt input) for several reasons. First, the acceptors are not dyes and therefore do not absorb radiation in the visible spectrum as well as dyes do. Second, the layers are formed by pressing techniques and, as such, require thicknesses which are far too large for efficient solar cells.
Multilayer photoelectric cells have been constructed from a phthalocyanine layer with or without an overcoat of malachite green, as reported, for example, in Topics in Current Chemistry, Springer-Verlag, Volume 61, 1976, page 124, and U.S. Pat. No. 3,789,216, issued Jan. 29, 1974. However, the conversion efficiency of such cells were very low--less than 10.sup.-4 percent, as reported in Springer-Verlag.
A layer of porphyrin or porphyrin-like material has also been used in the past to improve already existing solar cell semiconductors, such as selenium. Examples are disclosed in U.S. Pat. No. 3,935,031. However, only expensive inorganic semiconductors which themselves are self-sufficient cell materials have been suggested for such use with porphyrin.
Pyrylium and thiapyrylium dyes have been disclosed for use as sensitizers in photoconductive compositions, as noted, for example, in U.S. Pat. Nos. 3,938,994 and 3,997,342. No mention is made in these patents, however, as to the dye being useful with an adjacent layer of porphyrinic compound.
Other patents relating to the general background of organic solar cells include U.S. Pat. No. 3,912,931, issued Oct. 14, 1975.
Other patents relating to the general background of photoconductor compositions having a charge generating layer and a separate layer including a charge transport compound include U.S. Pat. Nos. 3,591,374, issued July 6, 1971; 3,837,851, issued Sept. 24, 1974; 3,840,368, issued Oct. 8, 1974; 3,996,049, issued Dec. 7, 1976; and 3,955,978, issued May 11, 1976.